The present application relates generally to correcting an alignment of a system of an aerial vehicle. For example, the present application relates to a method of and a system for aligning one or more components of a radar system, a heads-up display, an instrument landing system, a laser designation system, etc. mounted on an aerial vehicle.
For example, a radar system mounted on an aerial vehicle may be used to search a volume of space or ground for objects, to track detected objects that are moving, to identify certain types of objects, or to create an image of selected objects. Diverse types of objects include weather phenomena, moving objects such as cars, trucks, aircraft, missiles, satellites, fixed objects, etc. The quality of the data obtained from a radar system, as well as other aircraft systems utilizing orientation information, is generally related to the accuracy of the knowledge of the orientation of portions of the system's components such as a radar antenna(s) location and orientation in space particularly when the antenna is located on a moving platform. Thus, to provide the most effective radar operation, the radar antenna should be positioned at a known location and orientation with respect to the platform such as an aerial vehicle. Typically, a boresight of the radar antenna is defined relative to a radar antenna coordinate reference frame that is aligned with an Earth coordinate reference frame so that the antenna's location and orientation can be determined based on the movement and rotation of the moving platform.
Systems such as radar antennas can be commanded to steer electronically or mechanically in azimuth and elevation. For example, aircraft mounted weather radar systems use automatic antenna tilt control to command antenna azimuth scans at desired elevation angles relative to the horizon of the weather radar system. Additionally aircraft mounted weather radar systems can be commanded to scan in elevation. Using mechanical or electrical steering, the boresight of the radar antenna is moved to specific locations defined in the radar antenna coordinate reference frame. As a scan occurs, the aircraft orientation can change. The radar system can respond to the aircraft orientation change by receiving an indication of the aircraft orientation from sensors or other aircraft equipment. The radar system uses the received indication of the aircraft orientation to correct the antenna position, for example, so that the azimuth scan occurs across the horizon at a fixed elevation regardless of the aircraft orientation. Other systems, such as an instrument landing system, may not be steered, but their output data may require either some multiplicative or additive error correction based on an indication of the aircraft orientation.
Many factors can contribute to antenna alignment errors resulting from a deviation between a desired boresight angle and an actual boresight angle. For example, some errors arise during installation and mounting of the hardware. Installation and mounting errors tend to be relatively constant. As a result, after detection, constant errors generally can be removed through calibration. Other alignment errors are more dynamic, but may still arise from a change in aircraft state. A classic example of a systematic, but dynamic error seen on most aircraft is the change that occurs when the weight of the aircraft is supported by the wings instead of the wheels. The fuselage flexes between these two states. A vertical pointing angle of any sensor or instrumentation mounted at or forward of the cockpit changes as the fuselage flexes. Other examples include airframe deformation due to pressurization, uneven heating, and loading. Such airframe deformation alters the expected location and orientation of the antenna in a known and repeatable way. These variations can be said to be systematic.
Various techniques for aligning the antenna have been identified. For example, special test equipment can be used to mechanically align the antenna to the aircraft body coordinate reference frame. Additionally, adjustment of the antenna location and orientation can be performed in response to the observed performance of the radar system or other system being calibrated. For example, the antenna boresight of a radar can be aimed from a known position to a known fixed target position. The actual return from the known fixed target position is analyzed, and the antenna position is adjusted based on the analysis. According to another technique, the antenna can be positioned using optical tools. Other alignment systems may use data generated during normal operation of the sensor or instrumentation being calibrated. Such alignment systems may have difficulty tracking the rapid alignment changes induced by aircraft or environment state changes.
Such prior alignment techniques are time consuming and, in some circumstances, add unnecessary weight to the aircraft. Additionally, these techniques do not respond to dynamic and rapid alignment errors. Thus, there is a need for a system and a method for adjusting the position of an antenna for optimum radar or other such system performance without a need for additional time consuming procedures and without the need for additional mechanical components that add weight to the aircraft. Further still, there is a need for adjusting the position of an antenna to compensate for systematic, dynamic errors associated, for example, with deformation of the airframe.